1. Field of the Invention
The present invention relates generally to mitigating radio frequency (RF) signal interference in RF/digital receivers, and in particular to a selective filtering system and method for a global navigation satellite system (GNSS) susceptible to interference from signals in spectrally-adjacent bandwidths.
2. RF (Including GNSS) Background and Description of the Related Art
Global navigation satellite systems (GNSSs) include the Global Positioning System (GPS), which was established and is operated by the United States government and employs a constellation of 24 or more satellites in well-defined orbits at an altitude of approximately 20,200 km. These satellites continuously transmit microwave L-band radio signals in three frequency bands, centered at 1575.42 MHz, 1227.60 MHz and 1176.45 MHz, denoted as L1, L2 and L5 respectively. All GNSS signals include timing patterns relative to the satellite's onboard precision clock (which is kept synchronized by a ground station) as well as a navigation message giving the precise orbital positions of the satellites.
GNSS receivers are highly sensitive devices designed to receive very weak signals transmitted by the source satellites. GNSS receivers process the radio signals, computing ranges to the GNSS satellites, and by triangulating these ranges, the GNSS receiver determines its position and its internal clock error. Different levels of accuracies can be achieved depending on the observables used and the correction techniques employed. For example, accuracy within about 2 cm can be achieved using real-time kinematic (RTK) methods with single or dual-frequency (L1 and L2) receivers.
GNSS also includes Galileo (Europe), the GLObal NAvigation Satellite System (GLONASS, Russia), Beidou (China), Compass (proposed), the Indian Regional Navigational Satellite System (IRNSS) and QZSS (Japan, proposed). Galileo will transmit signals centered at 1575.42 MHz, denoted L1 or E1, 1176.45 denoted E5a, 1207.14 MHz, denoted E5b, 1191.795 MHz, denoted E5 and 1278.75 MHz, denoted E6. GLONASS transmits groups of FDM signals centered approximately at 1602 MHz and 1246 MHz, denoted GL1 and GL2 respectively. QZSS will transmit signals centered at L1, L2, L5 and E6. Groups of GNSS signals are herein grouped into “superbands.”
The United States' Global Positioning System (GPS) first reached fully operational capability on Jul. 17, 1995. After almost two decades, advances in technology and new demands have prompted efforts to modernize the GPS system. Part of the modernization are new civilian navigation signals to be transmitted on a frequency other than the L1 frequency (1575.42 MHz). This signal became known as the L2C signal because it is a civilian signal broadcast on the L2 frequency (1227.6 MHz). It is transmitted by all block IIR-M and newer generation satellites.
Whitehead et al. U.S. Pat. No. 6,744,404 shows an Unbiased Code Phase Estimator for Mitigating Multipath in GPS, and is incorporated herein by reference. U.S. Coast Guard Navigation Center, “GPS FAQ,” U.S. Department of Homeland Security; and Navstar Global Positioning System, “Interface Specification-ICD-GPS-200,” Navstar GPS Joint Program Office, are also incorporated herein by reference. Liu and Badke U.S. Provisional Patent Application Ser. No. 61/702,031 for GNSS System and Method Using Unbiased Code Phase Tracking with Interleaved Pseudo-Random Code is also incorporated herein by reference.
Dual-frequency GNSS receivers are preferably adaptable for use with all present and projected GNSS, transmitting signals which can be grouped into two “superbands” of radio signal frequencies generally in the range of about 1160 MHz to 1250 MHz and 1525 MHz to 1613 MHz.
RF signal frequency spectra allocations are highly regulated by the Federal Communications Commission (FCC) in the United States and by other agencies worldwide. As the airwaves become more crowded as a consequence of demand for RF signal spectra allocations, reception problems arise from signal interference.
For example, the telecommunications industry has experienced significant growth and increasing wireless traffic levels. Wireless telecommunications via RF signals are becoming increasingly popular among telecommunications service subscribers. To accommodate such demand, telecommunications service providers, through their industry associations, commonly seek FCC allocations of more frequency spectra.
The interests of the telecommunications industry are sometimes adverse to the other RF service providers. For example, GNSS service providers, including the U.S. Department of Defense with its Global Positioning System (GPS), are increasingly likely to encounter interference problems associated with nearby or spectrally-adjacent telecommunications bandwidth usage.
The present invention addresses the RF-digital signal interference problems with previous RF receivers. Heretofore, there has not been available an interference mitigating RF system and method with the advantages and features of the present invention.